The present invention relates to a method for shaping a green body of ceramic powder or, more particularly, to a method for shaping a green body of ceramic powder by use of a novel binder capable of giving improved mechanical strengths and shape retainability of the green body before firing as well as high smoothness of the surface thereof and also relates to a ceramic body obtained by firing the green body.
As is well known, so-called ceramics in the general categorization include an extremely wide diversity of materials in the types of materials as well as in the uses of the products. Irrespective of the type of the material, most of the ceramic products are manufactured according to the principle that a shaped body called agreen body is first shaped of a powder of the ceramic material and then the green body is fired to cause sintering of the particles of the ceramic powder. When the ceramic powder is selected, in particular, from various kinds of ferrites, titanates, alumina, cordierite, titanium dioxide, silicon carbide, silicon nitride and the like, problems are encountered in the insufficient workability of the ceramic powder in shaping and mechanical strengths and shape retainabililty of the green body so that it is a usual practice that the ceramic powder is blended with an organic binder with an object to improve the above mentioned properties.
The method for shaping a green body of the blend of the ceramic powder and the binder can be roughly classified into the wet process and the dry process practiced according to the kind of the binder and the type of the desired ceramic product. The binder suitable for the wet process is exemplified by methyl cellulose, carboxymethyl cellulose, starch and the like, among which methyl cellulose is used most widely. On the other hand, the dry process is undertaken by use of a binder such as polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, sodium alginate and the like, among which polyvinyl alcohol is preferred in the modern technology of ceramics.
When ceramic green bodies are shaped by extrusion molding in the wet process, metyl celluloses or carboxymethyl celluloses giving an aqueous solution of high viscosity are preferred as the binder from the standpoint of obtaining good shape retainability of the wet green body after extrusion although there is a problem of the insufficient mechanical strengths of the dried shaped body before firing. This problem is more serious when starch is used as the binder. When polyvinyl alcohols are used as the binder, considerable improvements can be obtained in the mechanical strengths of the dried green body but the shape retainability of the wet green body is poor because polyvinyl alcohols usually give an aqueous solution of relatively low viscosity. In addition, the surface of the extruded body is not always in good condition.
Alternatively, there has been proposed a method of shaping a green body of ceramic powder by extrusion using a non-aqueous solvent as the binding medium in place of water. Such a method of using an organic solvent is, however, disadvantageous and has no general applicability due to the problems of safety and environmental pollution as well as the increased costs in contradiction of the requirement of material saving. In the method of tape shaping as a modification of the wet process shaping of ceramic green bodies in a tape-like form, polyvinyl butyrals having relatively large binding power are sometimes used as a solution in an organic solvent though, naturally, not without the above mentioned problems inherent to the use of organic solvents.
In the dry process shaping of ceramic green bodies, the ceramic powder with the solution of a binder is spray-dried into beads or granules in which, as is mentioned above, polyvinyl alcohols are used usually as the binder. When a polyvinyl alcohol of a relatively large degree of polymerization is used as the binder in order to obtain a sufficient binding power, the slurry of the ceramic powder with the binder solution has poor sprayability and no uniform particle size distribution is obtained of the resultant ceramic granules. This defect can be mitigated by the use of a polyvinyl alcohol of a relatively low degree of polymerization though with sacrifice of the binding power. Methyl celluloses and carboxymethyl celluloses are still more unsuitable as the binder in the ceramic slurry to be dried by spraying due to the excessively high viscosity or consistency of the slurry despite the binding power thereof not always satisfactorily high.
Thus, any one of the conventional binders used for shaping ceramic green bodies has its own merits and demerits and is not satisfactory in all respects so that development of a ceramic binder having improved performance is eagerly desired in the technology of ceramics.